It's probably not you, it's them. Dwolla initially was putting new accounts such as you on a 30-day probationary period where funds cannot be sent to to a risky counterparty, such as a bitcoin exchange. What possibly happened is that the party you are attempting to trade with does a lot of trades and as a result falls into that high risk classification.
Perhaps you can find another party that hasn't recently used their Dwolla account for trading?
Here's a forum thread where you might find a trade:
- https://bitcointalksearch.org/topic/usd-between-mtgox-dwolla-78512